The present invention relates to interference pigments based on multiply coated platelet-shaped substrates.
Lustre pigments or special-effect pigments are employed in numerous fields in industry, especially in the sector of automotive finishes, in decorative coating, in plastics, in paints, in printing inks and in cosmetic formulations.
Lustre pigments which exhibit an angle-dependent colour change between two or more interference colours have a play of colour which makes them of particular interest for automotive finishes and in connection with counterfeit-protected documents of value. Pigments of this kind on the basis of multiply coated platelet-shaped substrates are known.
Interference pigments consist generally of platelet-shaped substrates with a thickness of from 200 to 1000 nm which are coated with highly refractive metal oxides or metal oxide mixtures with a thickness of from 50 to 300 nm. The optical properties of these pigments are critically determined by the refractive index of the metal oxide layer. In addition to the possibility of using chemical vapour deposition (CVD) or physical vapour deposition (PVD) techniques to prepare metal oxide layers having high densities and so refractive indices that lie close to the optimum, the deposition of metal oxides on finely divided, platelet-shaped substrates is frequently accomplished by titrating aqueous, usually acidic metal salt solutions against sodium hydroxide solution in the presence of a substrate, as described, for example, in DE 14 67 468 and DE 20 09 566.
A disadvantage of the vapour deposition technique is the high costs it entails. For instance, U.S. Pat. No. 4,434,010 discloses a multilayer interference pigment consisting of a central layer of a reflective metal, such as aluminium, and alternating layers of two transparent dielectric materials of high and low refractive index respectively, such as titanium dioxide and silicon dioxide, for example. This multilayer pigment is used preferably for counterfeit-protected securities.
JP H7-759 discloses a multilayer interference pigment with metallic lustre, for which a substrate is coated with alternate layers of titanium dioxide and silicon dioxide. The substrate comprises flakes of aluminium, gold or silver, or of mica or glass, with a coating of metals. The depth effect which is characteristic of and desired for interference pigments, however, cannot be generated. This is because of the total reflection of the light at the metal layer which forms the core. Consequently, the interference effect remains limited to the layers which are located on the metal layer. Furthermore, the lack of transparency of the substrate greatly restricts the diverse possibilities for combination with further pigments in applications-related formulations.
U.S. Pat. No. 3,438,796 and U.S. Pat. No. 5,135,812 describe, for example, metal lustre pigments having a central opaque aluminium film coated on both sides in alternation with dielectric films of low refractive index, such as silicon dioxide or magnesium fluoride, and with partially transparent metal films, such as films of chromium or aluminium, for example. Owing to the preparation process, the central metal film of these pigments is coated only on the top and bottom sides of the platelets, while the side areas constitute broken edges and lie open towards the medium.
DE 44 05 494, DE 44 37 753, DE 195 16 181 and DE 195 15 988 disclose lustre pigments prepared by coating metal platelets, especially aluminium flakes, with metal oxide layers of low refractive index, such as with a silicon dioxide layer, and with non-selectively absorbing metal oxide layers or metal layers of high refractive index, using CVD or wet-chemical techniques.
Lustre pigments based on metal substrates frequently have good performance properties, including good opacity, but the result on application, such as in the paint, for example, is a xe2x80x9chardxe2x80x9d metallic lustre, which is frequently unwanted.
Lustre pigments based on transparent platelet-shaped substrates which do not have this xe2x80x9chardxe2x80x9d metallic lustre are the subject of WO 93/12182. Mica flakes are covered with a metal oxide layer of high refractive index, such as TiO2, and with a non-selectively absorbing layer. Depending on the thickness of the TiO2 layer, when viewed straight on these pigments exhibit a particular interference colour which becomes increasingly weaker as the viewing angle becomes more oblique and which finally flips to grey or black. The interference colour does not change, but a decrease is found in the colour saturation.
JP 1992/93206 claims lustre pigments on the basis of glass flakes or mica particles which are covered with an opaque metal layer and with alternating layers of SiO2 and TiO2.
EP 0 753 545 discloses lustre pigments based on multiply coated, nonmetallic, platelet-shaped substrates which are of high refractive index, are at least partially transparent to visible light and have at least one layer assembly comprising a colourless coating of low refractive index and a reflective coating which absorbs selectively or nonselectively. Disadvantages of this invention are the technically very complex and costly preparation process and the frequent difficulty in reproducing the pigments in the desired product quality.
The object of the present invention is to provide an essentially transparent interference pigment having strong interference colours and/or a strong angular dependence of the interference colours which is notable for its advantageous performance properties and which at the same time can be prepared in a simple manner.
Surprisingly, an interference pigment has now been found which is based on multiply coated, platelet-shaped substrates and comprises a particular arrangement of optically functional layers by means of which particular optical effects are achieved.
The invention therefore provides interference pigments on the basis of multiply coated, platelet-shaped substrates which comprise at least one layer sequence comprising
(A) a coating having a refractive index nxe2x89xa72.0,
(B) a colourless coating having a refractive index nxe2x89xa61.8, and
(C) a nonabsorbing coating of high refractive index,
and, if desired,
(D) an external protective layer.
The invention also provides for the use of the pigments of the invention in paints, lacquers, printing inks, plastics, ceramic materials, glasses and cosmetic formulations.
Suitable base substrates for the multilayer pigments of the invention are firstly opaque and secondly transparent platelet-shaped substances. Preferred substrates are phyllosilicates and metal oxide-coated, platelet-shaped materials. Of particular suitability are natural and synthetic micas, talc, kaolin, platelet-shaped iron oxides, bismuth oxychloride, flakes of glass, SiO2, Al2O3 or TiO2, synthetic ceramic flakes, carrier-free synthetic platelets, LCPs or other comparable materials.
The size of the base substrates per se is not critical and can be matched to the particular target application. In general, the platelet-shaped substrates have a thickness of between 0.1 and 5 xcexcm, in particular between 0.2 and 4.5 xcexcm. The extent in the two other dimensions is usually between 1 and 250 xcexcm, preferably between 2 and 200 xcexcm and, in particular, between 5 and 50 xcexcm.
The thickness of the individual layers of high and low refractive index on the base substrate is essential for the optical properties of the pigment. For a pigment with intensive interference colours, the thickness of the individual layers must be adjusted precisely with respect to one another.
If n is the refractive index of a thin layer and d its thickness, the interference colour of this layer is defined by the product nxc2x7d(nxc2x7d=optical thickness). The colours which result from such a film under perpendicular light incidence in reflected light result from an intensification of the light of wavelength   λ  =            4                        2          ⁢          N                -        1              ·    n    ·    d  
and by an attenuation of the light of wavelength   λ  =            2      N        ·    n    ·    d  
where N is a positive integer.
The variation in colour which results with increasing film thickness is a consequence of the intensification or attenuation of certain light wavelengths through interference. If two or more layers in a multilayer pigment possess the same optical thickness, the colour of the reflected light becomes more intense as the number of layers increases. In addition to this, it is possible through an appropriate choice of layer thicknesses to achieve a particularly strong variation of the colour as a function of the viewing angle. A pronounced, so-called colour flop is developed. The thickness of the individual metal oxide layers, irrespective of their refractive index, depends on the field of use and is generally from 10 to 1000 nm, preferably from 15 to 800 nm and, in particular, 20-600 nm.
The lustre pigments of the invention feature a coating (A) of high refractive index in combination with a colourless coating (B) of low refractive index and located thereon a nonabsorbing coating of high refractive index. The pigments can comprise two or more, identical or different combinations of layer assemblies, although preference is given to covering the substrate with only one layer assembly (A)+(B)+(C). In order to make the colour flop more intense the pigment of the invention may comprise up to 4 layer assemblies, although the thickness of all of the layers on the substrate should not exceed 3 xcexcm.
The layer (A) of high refractive index has a refractive index nxe2x89xa72.0, preferably nxe2x89xa72.1. Materials suitable as the layer material (A) are all materials known to the skilled worker which are of high refractive index, are filmlike and can be applied permanently to the substrate particles. Particularly suitable materials are metal oxides or metal oxide mixtures, such as TiO2, Fe2O3, ZrO2, ZnO or SnO2, or compounds of high refractive index such as, for example, iron titanates, iron oxide hydrates, titanium suboxides, chromium oxide, bismuth vanadate, cobalt aluminate, and also mixtures or mixed phases of these compounds with one another or with other metal oxides.
The thickness of the layer (A) is 10-550 nm, preferably 15-400 nm and, in particular, 20-350 nm.
Colourless materials of low refractive index suitable for the coating (B) are preferably metal oxides or the corresponding oxide hydrates, such as SiO2, Al2O3, AlO(OH), B2O3 or a mixture of these metal oxides. The thickness of the layer (B) is 10-1000 nm, preferably 20-800 nm and, in particular, 30-600 nm.
Materials particularly suitable for the non-absorbing coating (C) of high refractive index are colourless metal oxides such as TiO2, ZrO2, SnO2, ZnO and BiOCl, and also mixtures thereof. The thickness of the layer (C) is 10-550 nm, preferably 15-400 nm and, in particular, 20-350 nm.
In addition to the standard layer assembly (A)+(B)+(C), which may be present up to four times in the pigment of the invention, there are other preferred embodiments. For instance, between the substrate (S) and the layer (A), between the layer (A) and (B), between layer (B) and (C) and/or between layer (C) and the top layer (D) the pigment of the invention may have a further absorbing or nonabsorbing layer [(S1), (A1), (B1), (C1)]. The thickness of the interlayers is 1-50 nm, preferably 1-40 nm and, in particular, 1-30 nm.
A particularly preferred embodiment is the coating of the substrate with the following layer assembly:
(S1) optional, SnO2 
(A) TiO2 or Fe2O3 
(B) SiO2 
(B1) optional, SnO2 
(C) TiO2 
(D) final coating related to application
Coating the substrates with layers (A) and (C) of high refractive index, a layer (B) of low refractive index and, if desired, further coloured or colourless coatings produces pigments whose colour, gloss, opacity and angular dependence of perceived colour can be varied within wide limits.
The pigments of the invention are easy to produce by virtue of the generation of two or more interference layers of high and low refractive index, precisely defined thickness and smooth surface on the finely divided, platelet-shaped substrates.
The metal oxide layers are preferably applied by wet-chemical means, it being possible to use the wet-chemical coating techniques developed for the production of pearl lustre pigments; techniques of this kind are described, for example, in DE 14 67 468, DE 19 59 988, DE 20 09 566, DE 22 14 545, DE 22 15 191, DE 22 44 298, DE 23 13 331, DE 25 22 572, DE 31 37 808, DE 31 37 809, DE 31 51 343, DE 31 51 354, DE 31 51 355, DE 32 11 602, DE 32 35 017 or else in further patent documents and other publications.
In the case of wet coating, the substrate particles are suspended in water, and one or more hydrolysable metal salts are added at a pH which is appropriate for hydrolysis and is chosen such that the metal oxides or metal oxide hydrates are precipitated directly onto the platelets without any instances of secondary precipitation. The pH is kept constant usually by simultaneous metered addition of a base and/or acid. Subsequently, the pigments are separated off, washed and dried and, if desired, are calcined, it being possible to optimize the calcination temperature in respect of the particular coating present. In general, the calcination temperatures are between 250 and 1000xc2x0 C., preferably between 350 and 900xc2x0 C. If desired, following the application of individual coatings the pigments can be separated off, dried and, if desired, calcined before being resuspended for the application of further layers by precipitation.
Coating can also take place in a fluidized-bed reactor by means of gas-phase coating, in which case it is possible, for example, to make appropriate use of the techniques proposed in EP 0 045 851 and EP 0 106 235 for preparing pearl lustre pigments.
The metal oxide of high refractive index used is preferably titanium dioxide and/or iron oxide, and the metal oxide of low refractive index preferably used is silicon dioxide.
For the application of the titanium dioxide layers, preference is given to the technique described in U.S. Pat. No. 3,553,001.
An aqueous titanium salt solution is added slowly to a suspension, heated to about 50-100xc2x0 C., of the material to be coated, and a substantially constant pH of about 0.5-5 is maintained by simultaneous metered addition of a base, for example aqueous ammonia solution or aqueous alkali metal hydroxide solution. As soon as the desired layer thickness of the TiO2 precipitate has been reached, the addition of both titanium salt solution and base is terminated.
This technique, also referred to as the titration process, is notable for the fact that it avoids an excess of titanium salt. This is achieved by supplying to the hydrolysis only that quantity per unit time which is necessary for uniform coating with the hydrated TiO2 and which can be received per unit time by the available surface area of the particles to be coated. There is therefore no production of hydrated titanium dioxide particles not precipitated on the surface to be coated.
The application of the silicon dioxide layers can be performed, for example, as follows. A potassium or sodium silicate solution is metered into a suspension, heated to about 50-100xc2x0 C., of the substrate that is to be coated. The pH is held constant at about 6-9 by simultaneous addition of a dilute mineral acid, such as HCl, HNO3 or H2SO4. As soon as the desired layer thickness of SiO2 has been reached, the addition of the silicate solution is terminated. The batch is subsequently stirred for about 0.5 h.
In order to enhance the light stability and weather stability it is frequently advisable to subject the finished pigment to an aftercoating or after-treatment process, depending on the field of use. Suitable such processes are those described, for example, in DE-C 22 15 191, DE-A 31 51 354, DE-A 32 35 017 or DE-A 33 34 598. Such aftercoating further increases the chemical stability or facilitates the handling of the pigment, especially its incorporation into different media.
The pigments of the invention are compatible with a large number of colour systems, preferably from the sector of lacquers, paints and printing inks, especially security printing inks. Owing to the uncopyable optical effects, the pigments of the invention can be used in particular for producing counterfeit-protected documents of value, such as bank notes, cheques, cheque cards, credit cards, identity cards, etc. In addition, the pigments are also suitable for the laser marking of paper and plastics and for applications in the agricultural sector, such as for glasshouse films, for example.
The invention therefore also provides for the in use of the pigments in formulations such as paints, printing inks, lacquers, plastics, ceramic materials and glasses and for cosmetics preparations.
It is of course the case that for the various target applications the multilayer pigments can also be employed advantageously in blends with other pigments, examples being transparent and hiding white, coloured and black pigments, and with platelet-shaped iron oxides, organic pigments, holographic pigments, LCPs (liquid crystal polymers) and conventional transparent, coloured and black lustre pigments based on metal oxide-coated mica and SiO2 platelets, etc. The multilayer pigments can be mixed in any proportion with customary commercial pigments and extenders.